1. Field of the Invention
Exemplary embodiments of the present invention relate to an optical detection apparatus and method in a Wavelength Division Multiplexing-Passive Optical Network (WDM-PON) based on a laser seed lightwave; and, more particularly, to an optical detection apparatus and method in a WDM-PON, which detects an electrical signal by controlling the phase and polarization of seed lightwaves and upward signal lights in real time (First embodiment), detects an electrical signal by controlling only the phase of seed lightwaves and upward signal lights in real time (Second embodiment), and detects an electrical signal without controlling the phase and polarization of seed lightwaves and upward signal lights in real time (Third embodiment).
2. Description of Related Art
The WDM-PON refers to a network which connects an optical line terminal (OLT) of a central office (CO) of a communication service provider to optical network units (ONU) of subscribers through passive optical elements, and distributes and transmits an optical signal including a variety of multiplexed data to the respective ONUs.
Recently, a WDM-PON system based on a laser seed lightwave has been actively researched. The WDM-PON system is implemented in such a manner that the CO uses a laser light source as a seed lightwave and the subscribers modulate and amplify an upward signal by using a reflective modulator such as a reflective semiconductor optical amplifier (R-OSA).
Meanwhile, a coherent detection scheme refers to technology which mixes signal light with reference light, of which the wavelength, polarization, and phase coincide with those of the signal light, such that a photodiode detects an optical current. Conventionally, it has been known that the implementation of the coherent detection scheme is very difficult and requires a lot of costs. Therefore, the coherent detection scheme is not suitable for being used as subscriber network technology. Accordingly, a direct detection scheme has been frequently used, which measures the total amount of light.
Meanwhile, a balanced coherent receiver is frequently used in the coherent detection scheme.
FIG. 5 is a diagram explaining an example of a general balanced coherent receiver.
Referring to FIG. 5, when reference light Eref=Aref cos(ωt+φref) and signal light ES=AS cos(ωt+φS) are inputted to the balanced coherent receiver, a detection current I(t)≈2AsARef cos(φS−φRef) is outputted.
In general, however, the system performance of the WDM-PON system is considerably limited by light reflection in lines between the CO and the subscribers. The WDM-PON system using the above-described direct detection scheme has a problem that it does not guarantee accurate optical detection performance, because it performs the optical detection by simply adding seed lightwave and upward signal light. The problem will be described in detail as follows.
When a seed lightwave ESeed(t)=ASeed cos(ωt+φSeed) having a wavenumber of ω is used, upward signal light obtained by modulating the seed lightwave may be expressed as
                    E        S            ⁡              (        t        )              =                  ∑        i            ⁢                        A                      m            ,            i                          ⁢                  cos          ⁡                      (                                          ω                                  m                  ,                  i                                            ⁢              t                        )                          ⁢                  cos          ⁡                      (                                          ω                ⁢                                                                  ⁢                t                            +                              ϕ                S                                      )                                ,and reflected light may be expressed as
            E      R        ⁡          (      t      )        =            ∑      i        ⁢                  A                  R          ,          i                    ⁢                        cos          ⁡                      (                                          ω                ⁢                                                                  ⁢                t                            +                              ϕ                                  R                  ,                  i                                                      )                          .            Here, ωm,i, represents a basis frequency when the upward modulated signal is Fourier-expanded, Am,i represents the magnitude of ωm,i,and φS has a constant value. Furthermore, φR,i represents a phase of the reflected light having a random value, and AR,i represents the intensity of the reflected light having the phase of φR,i. In the conventional direct detection scheme, when two signals are added and inputted to the photodiode, a detection current may be expressed as Equation 1 below.
                                                                        I                ⁢                                  (                  t                  )                                            ≈                            ⁢                                                [                                                                                    ∑                        i                                            ⁢                                                                        A                                                      m                            ,                            i                                                                          ⁢                                                  cos                          ⁡                                                      (                                                                                          ω                                                                  m                                  ,                                  i                                                                                            ⁢                              t                                                        )                                                                          ⁢                                                  cos                          ⁡                                                      (                                                                                          ω                                ⁢                                                                                                                                  ⁢                                t                                                            +                                                              ϕ                                S                                                                                      )                                                                                                                +                                                                  ∑                        j                                            ⁢                                                                        A                                                      R                            ,                            j                                                                          ⁢                                                  cos                          ⁡                                                      (                                                                                          ω                                ⁢                                                                                                                                  ⁢                                t                                                            +                                                              ϕ                                j                                                                                      )                                                                                                                                ]                                2                                                                                        =                            ⁢                              const                +                                                      ∑                    i                                    ⁢                                                                                    A                                                  m                          ,                          i                                                2                                            ⁡                                              [                                                  cos                          ⁡                                                      (                                                                                          ω                                                                  m                                  ,                                  i                                                                                            ⁢                              t                                                        )                                                                          ]                                                              2                                                  +                                                      ∑                    j                                    ⁢                                      A                                          R                      ,                      j                                        2                                                  +                                                                                                      ⁢                                                2                  ⁢                                                            ∑                                              i                        ,                        j                                                              ⁢                                                                  A                                                  R                          ,                          i                                                                    ⁢                                              A                                                  R                          ,                          j                                                                    ⁢                                              cos                        ⁡                                                  (                                                                                    ϕ                              i                                                        -                                                          ϕ                              j                                                                                )                                                                                                                    +                                                                                                      ⁢                                                ∑                                      i                    ,                    j                                                  ⁢                                                      A                                          m                      ,                      j                                                        ⁢                                      A                                          R                      ,                      j                                                        ⁢                                      cos                    ⁡                                          (                                                                        ω                                                      m                            ,                            i                                                                          ⁢                        t                                            )                                                        ⁢                                      cos                    ⁡                                          (                                                                        ϕ                          S                                                -                                                  ϕ                          j                                                                    )                                                                                                                              Eq        .                                  ⁢        1            
In Equation 1,
      ∑    i    ⁢                    A                  m          ,          i                2            ⁡              [                  cos          ⁡                      (                                          ω                                  m                  ,                  i                                            ⁢              t                        )                          ]              2  is a signal component,
            ∑      j        ⁢          A              R        ,        j            2        +      2    ⁢                  ∑        i            ⁢                        A                      R            ,            i                          ⁢                  A                      R            ,            j                          ⁢                  cos          ⁡                      (                                          ϕ                i                            -                              ϕ                j                                      )                              is a DC component by the reflected light, and
      ∑          i      ,      j        ⁢            A              m        ,        j              ⁢          A              R        ,        j              ⁢          cos      ⁡              (                              ω                          m              ,              i                                ⁢          t                )              ⁢          cos      ⁡              (                              ϕ            S                    -                      ϕ            j                          )            is a component by the interference between the reflected light and the signal light.
FIG. 6 is a diagram explaining an RF power spectrum according to a general modulation scheme. FIG. 6 shows that the RF power spectrum changes according to the modulation scheme.
Referring to FIG. 6, when NRZ coding is performed, a DC component of a signal component increases. Accordingly, the signal component and a reflected light component cannot be separated.
Furthermore, referring to FIG. 6, when Manchester coding with no DC component as an upward signal is used,
      ∑    i    ⁢                              A                      m            ,            i                    2                ⁡                  [                      cos            ⁡                          (                                                ω                                      m                    ,                    i                                                  ⁢                t                            )                                ]                    2        ⁢                  ⁢    and    ⁢                  ⁢                  ∑                  i          ,          j                    ⁢                        A                      m            ,            i                          ⁢                  A                      R            ,            j                          ⁢                  cos          ⁡                      (                                          ω                                  m                  ,                  i                                            ⁢              t                        )                          ⁢                  cos          ⁡                      (                                          ϕ                S                            -                              ϕ                j                                      )                              having a signal component and
            ∑      j        ⁢          A              R        ,        j            2        +      2    ⁢          ∑                        A                      R            ,            i                          ⁢                  A                      R            ,            j                          ⁢                  cos          ⁡                      (                                          ϕ                i                            -                              ϕ                j                                      )                              having no signal component may be discriminated by an electrical filter. In this method, however,
      ∑          i      ,      j        ⁢            A              m        ,        i              ⁢          A              R        ,        j              ⁢          cos      ⁡              (                              ω                          m              ,              i                                ⁢          t                )              ⁢          cos      ⁡              (                              ϕ            S                    -                      ϕ            j                          )            having a signal component may act as noise depending on the phase φj of the reflected light and the phase φS of the signal light, while the Manchester coding removes much noise in comparison with the NRZ coding. That is, the conventional direct detection method does not remove noise caused by the reflected light perfectly.
In other words, the conventional method has a problem in that it does not remove noise caused by the reflected light perfectly. To solve such a problem, the present invention has been proposed.